There is known a method for ultrasonic inspection of materials (cf. A. K. Gurvich, I. N. Yermolov, "Ultrazvukovoy control svarnykh shvov"/"Ultrasonic Inspection of Welded Joints"/, Technika Publishers, Kiev, 1972, pp. 66-69), which comprises emitting ultrasonic vibrations into a material, receiving and processing the reflected ultrasonic vibrations, setting a noise cut-off level, separating signals from noise at a preset level and recording the results of the inspection. According to the method under review, the noise cut-off level is set manually by the operator who observes the noise level in the maximum sensitivity zone.
There is known a device for effecting the method under review, wherein the output of an ultrasonic generator, whose input is connected to one of the outputs of a synchronizer, is connected to the input of an ultrasonic transducer which is acoustically coupled to a material to be inspected; the output of the ultrasonic transducer is electrically coupled via a unit for receiving and processing reflected ultrasonic vibrations and a unit for separating signals from noise, which are placed in series, to a first input of a signals recording unit whose second input is connected to a second output of the synchronizer. In the device under review, the unit for separating signals from noise is a noise cut-off detector built around semiconductor diodes; the noise cut-off level is determined by the value of reference voltage which is set manually by the operator in accordance with the noise level in the material under inspection.
Noise levels may differ considerably even in adjacent areas of an article being tested, which is due to the structural non-uniformity of the material, as well as an inadequate acoustic contact.
Such situations often occur when inspecting articles of a complex shape and variable thickness, manufactured from some of the recently developed alloys with their varible structural noise levels. Reactors and pipelines of atomic power stations may serve as an example.
Manual setting of the noise cut-off level in the known device does not make it possible to rapidly change that level following a change in the noise level in the material being tested.
Furthermore, an optimum sensitivity is never reached because the operator normally sets the noise cut-off level at a point somewhat higher than the maximum noise level.
Due to the subjective selection of the noise cut-off level, this level may be higher than necessary; as a result, some flaws may remain undetected. The noise cut-off level may also be lower than necessary, which may lead to a detection of false defects. In both cases the inspection lacks confidence.
The rate of inspection of articles having portions with different structural noise levels is quite low because at each such portion one must readjust the noise cut-off level; if the structural noise level of a portion differs from that of the previous portion, the former portion must be inspected twice.
The device under review has to be readjusted each time it is used to inspect a different article with a significantly different structural noise level. However, such a readjustment is a complicated and arduous task.